JP7722306B2 - Pressure-sensitive adhesive composition and pressure-sensitive adhesive sheet - Google Patents

Description

The present invention relates to a pressure-sensitive adhesive composition and a pressure-sensitive adhesive sheet.

Adhesive sheets with an adhesive layer formed from a pressure-sensitive adhesive are easier to handle than adhesives, and are therefore used in a wide range of fields as display labels and fixing tapes. They can be broadly divided into high-adhesion types that provide permanent fixation, and removable types that allow the adhesive sheet to be peeled off after use. There are various resin-based adhesives, with acrylic-based adhesives being the most widely used. Acrylic-based adhesives have the advantage that basic adhesive properties such as adhesive strength and tack can be easily controlled by adjusting the monomer composition and compatible additives.

As a removable acrylic adhesive, Patent Document 1 discloses an adhesive composition characterized by an acrylic adhesive layer formed by crosslinking an acrylic polymer composed of a (meth)acrylic ester ester having an alkyl group containing 4 to 12 carbon atoms as the main monomer, copolymerized with at least 3 to 20% by weight of a carboxyl-containing radically polymerizable monomer, with a gel fraction of 80% by weight or more, using a crosslinker having a functional group reactive with carboxyl groups within the molecule. Patent Document 2 also discloses a removable adhesive sheet characterized by a pressure-sensitive adhesive layer formed by coating one side of a substrate with a pressure-sensitive adhesive composition comprising heat-expandable microspheres, an acrylic adhesive with a weight-average molecular weight of 100,000 to 300,000 and an acid value of 30 or more that is reactive with the crosslinker, a tackifier resin with a softening point of 90°C or higher and a solubility parameter (SP value) of 8.9 or higher, and a crosslinker.

Japanese Patent Application Laid-Open No. 2000-328016 Japanese Patent Application Laid-Open No. 2003-160765

However, the adhesive composition of Patent Document 1 has the problem that when the adhesive sheet is peeled off after being stored attached to an adherend under high temperature or humid heat conditions for an extended period of time, the adhesive remains on the adherend. Furthermore, the adhesive sheet of Patent Document 2 has difficulty ensuring sufficient removability at room temperature, and suffers from peeling problems due to reduced adhesive strength at high temperatures, as well as insufficient removability after long-term storage at high temperatures.

The problem that the present invention aims to solve is to provide an adhesive composition and adhesive sheet that has sufficient adhesive strength and high tackiness, exhibits good removability even when applied to highly polar substrates such as SUS or steel plates that tend to leave residual adhesive when removed, and maintains sufficient removability even when stored in an applied state under high temperature or humid heat conditions for extended periods; and also provides an adhesive composition and adhesive sheet that, when applied, shows little adhesive protrusion from the sheet even under high temperature and high load conditions, and can be stored stacked on heavy substrates such as steel plates, even at high temperatures.

In order to solve the above-mentioned problems, the present inventors have discovered a pressure-sensitive adhesive composition comprising an acrylic acid ester copolymer, which is a copolymer of a monomer mixture consisting of (A) a (meth)acrylic acid alkyl ester monomer having 1 to 12 carbon atoms in the alkyl group, (B) a monomer having an acidic group, and (C) a monomer having a hydroxyl group, and an isocyanate-based curing agent, wherein the content of the acidic group-containing monomer (B) is 0.01% by mass or more and 6% by mass or less, and the content of the hydroxyl group-containing monomer (C) is 0.05% by mass or more and 10% by mass or less, based on 100% by mass of the monomer mixture, and the gel fraction is 55% or more.

That is, the present invention relates to the following [1] to [4].

[1] The present invention relates to a pressure-sensitive adhesive composition comprising an acrylic acid ester copolymer, which is a copolymer of a monomer mixture consisting of a (meth)acrylic acid alkyl ester monomer (A) having 1 to 12 carbon atoms in the alkyl group, a monomer (B) having an acidic group, and a monomer (C) having a hydroxyl group, and an isocyanate-based curing agent, wherein the content of the monomer (B) having an acidic group is 0.01% by mass or more and 6% by mass or less, relative to 100% by mass of the monomer mixture, and the content of the monomer (C) having a hydroxyl group is 0.05% by mass or more and 10% by mass or less, relative to 100% by mass of the monomer mixture, and the gel fraction is 55% or more.

[2] The pressure-sensitive adhesive composition according to the above item [1], wherein the content of the (meth)acrylic acid alkyl ester monomer (a1) having an alkyl group with 8 to 12 carbon atoms in 100% by mass of the (meth)acrylic acid alkyl ester monomer (A) having an alkyl group with 1 to 12 carbon atoms is 55% by mass or more.

[3] The pressure-sensitive adhesive composition according to the above [1] or [2], wherein the ratio (P) of the loss modulus at 25°C to the loss modulus at 150°C measured at 1 Hz, as represented by Equation 1, is 0.02 or more and 0.3 or less.

P = (loss modulus at 150°C when measured at 1 Hz) / (loss modulus at 25°C when measured at 1 Hz) (Equation 1)

[4] A pressure-sensitive adhesive sheet comprising a substrate and a pressure-sensitive adhesive layer comprising a cured product of the pressure-sensitive adhesive composition described in any one of [1] to [3] above.

The present invention provides an adhesive composition and adhesive sheet that have sufficient adhesive strength and high tackiness, exhibit good removability even when applied to highly polar substrates such as SUS and steel plates, which tend to leave residual adhesive residue when removed, and maintain sufficient removability even when stored in an applied state under high temperature or humid heat conditions for extended periods. It also provides an adhesive composition and adhesive sheet that exhibits minimal adhesive extrusion from the sheet even when applied to high temperature and high load conditions, and can be stored stacked on heavy substrates such as steel plates, even at high temperatures.

The present invention is described in detail below. It goes without saying that other embodiments are also included within the scope of the present invention as long as they are consistent with the spirit of the present invention. Furthermore, in this specification, numerical ranges specified using "to" include the numerical values before and after "to" as the range's lower and upper limits.

In this specification, unless otherwise specified, the terms "(meth)acrylic acid" and "(meth)acrylate" refer to "acrylic acid or methacrylic acid" and "acrylate or methacrylate," respectively. Furthermore, "(meth)acrylic acid ester monomer" is a general term for "acrylic acid ester monomer" and "methacrylic acid ester monomer." Furthermore, a (meth)acrylic acid alkyl ester monomer (A) having an alkyl group with 1 to 12 carbon atoms may be referred to simply as "monomer (A)," a monomer (B) having an acidic group may be referred to simply as "monomer (B), and a monomer (C) having a hydroxyl group may be referred to simply as "monomer (C)."

In this specification, "Mw" refers to the weight-average molecular weight in terms of polystyrene determined by gel permeation chromatography (GPC). "Mn" refers to the number-average molecular weight in terms of polystyrene determined by GPC. These can be measured by the method described in the Examples section.

Unless otherwise noted, the various components mentioned in this specification may be used independently, either alone or in combination of two or more.

<Acrylate ester copolymer>
The acrylic acid ester copolymer means a polymer of a monomer having an ethylenically unsaturated bond, such as a (meth)acrylic acid ester monomer.
The acrylic ester copolymer of the present invention is a copolymer of a monomer mixture comprising at least a (meth)acrylic acid alkyl ester monomer (A) having 1 to 12 carbon atoms in the alkyl group, a monomer (B) having an acidic group, and a monomer (C) having a hydroxyl group, wherein the content of the monomer (B) having an acidic group is 0.01% by mass or more and 6% by mass or less, based on 100% by mass of the monomer mixture, and the content of the monomer (C) having a hydroxyl group is 0.05% by mass or more and 10% by mass or less, based on 100% by mass of the monomer mixture.

The monomers are classified into (meth)acrylic acid alkyl ester monomers (A) in which the alkyl group has 1 to 12 carbon atoms, monomers (B) having an acidic group, monomers (C) having a hydroxyl group, and other monomers. As the monomers constituting the acrylic copolymer, monomers (A), (B), and (C) are essential components, and other monomers may be used as necessary.

[Monomer (A)]
Monomer (A) is a (meth)acrylic acid alkyl ester monomer having 1 to 12 carbon atoms in the alkyl group. (Meth)acrylic acid alkyl ester monomers having 1 to 12 carbon atoms in the alkyl group are classified into (meth)acrylic acid alkyl ester monomers (a1) having 8 to 12 carbon atoms in the alkyl group and (meth)acrylic acid alkyl ester monomers (a2) having 1 to 7 carbon atoms in the alkyl group. It is preferable to use (meth)acrylic acid alkyl ester monomers (a1) having 8 to 12 carbon atoms in the alkyl group, and it is more preferable to use (a1) and (a2) in combination. By using (meth)acrylic acid alkyl ester monomers (a1) having 8 to 12 carbon atoms in the alkyl group, it is possible to achieve both high levels of tackiness and removability.

Examples of the monomer (a1) include octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, etc. Among these, 2-ethylhexyl acrylate and lauryl (meth)acrylate are preferred from the viewpoint of removability.

The content of monomer (a1) is not particularly limited, but is preferably 55% by mass or more, and more preferably 65% by mass or more, based on 100% by mass of the monomer mixture constituting the acrylic acid ester copolymer. There is no particular upper limit, but if one must be set, it is preferably 90% by mass or less, and more preferably 80% by mass or less. By keeping it within this range, it is possible to achieve both high tackiness and removability.

Examples of the monomer (a2) include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, etc. Among these, butyl acrylate and (iso)butyl acrylate are preferred because they exhibit good adhesive strength to various adherends.

[Monomer (B)]
The monomer (B) is a monomer having an acidic group. By using the monomer (B), it is possible to exhibit good adhesive strength and cohesive strength to various adherends.

Examples of the monomer (B) include (meth)acrylic acid, 2-carboxyethyl (meth)acrylate, 2-(meth)acryloyloxyethyl succinic acid, 2-(meth)acryloyloxyethyl hexahydrophthalic acid, 2-(meth)acryloyloxyethyl phthalic acid, crotonic acid, maleic acid, itaconic acid, fumaric acid, citraconic acid, mesaconic acid, and other carboxylic acids having an unsaturated bond; and 2-(meth)acryloyloxyethyl acid phosphate. From the viewpoints of the cohesive strength of the resulting pressure-sensitive adhesive composition and copolymerizability with other monomers, (meth)acrylic acid and 2-carboxyethyl (meth)acrylate are preferred.

The content of the monomer (B) is from 0.01% by mass to 6% by mass, preferably from 0.1% by mass to 4% by mass, more preferably from 0.5% by mass to 3.5% by mass, and more preferably from 1% by mass to 3.5% by mass, based on 100% by mass of the monomer mixture constituting the acrylic acid ester copolymer.
% by mass or less is particularly preferred. When the content of monomer (B) is 0.05% by mass or more, high adhesive strength and cohesive strength can be imparted. Furthermore, when the content of monomer (B) is 6% by mass or less, adhesion of the PSA composition to the adherend can be suppressed even after the PSA sheet is peeled from the adherend.

[Monomer (C)]
The monomer (C) is a monomer having a hydroxyl group. The use of the monomer (C) serves as a reaction site with the isocyanate-based curing agent, thereby imparting good removability even under high temperature or humid heat conditions.

Examples of monomers having a hydroxyl group include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, and hydroxy(meth)acrylamides such as hydroxymethyl acrylamide and hydroxyethyl acrylamide. From the standpoint of reactivity with isocyanate-based curing agents and the resulting cohesive strength, 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate are preferred.

The content of monomer (C) is 0.05% by mass or more and 10% by mass or less, preferably 0.07% by mass or more and 1.8% by mass or less, and more preferably 0.08% by mass or more and 1% by mass or less, based on 100% by mass of the monomer mixture constituting the acrylic acid ester copolymer. A content of monomer (C) of 0.05% by mass or more can impart high cohesive strength during the curing agent reaction. A content of monomer (B) of 10% by mass or less can suppress a significant increase in the viscosity of the solution after the curing agent is added, and can impart good removability.

[Other monomers]
The other monomer is a monomer other than the monomer (A), the monomer (B), and the monomer (C). The other monomer is not particularly limited, and can be appropriately selected from monomers copolymerizable with monomer (A), monomer (B), and monomer (C). Examples of the other monomer include vinyl carboxylate monomers such as vinyl acetate, vinyl propionate, and vinyl butyrate; other vinyl monomers such as styrene and acrylonitrile; monomers having an alicyclic structure such as isobornyl (meth)acrylate and cyclohexyl (meth)acrylate; monomers having an aromatic ring such as phenyl (meth)acrylate, phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, and biphenyl (meth)acrylate; monomers having an alkoxy(poly)alkylene oxide such as 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, methoxypolyethylene glycol (meth)acrylate, methoxypolypropylene glycol (meth)acrylate, and ethoxypolypropylene glycol (meth)acrylate; acid anhydrides having a polymerizable unsaturated bond in the molecule such as maleic anhydride and itaconic anhydride; monomers having an amino group; other monomers having a nitrogen atom; monomers having an epoxy group; and monomers having an isocyanato group.

Examples of monomers having an amino group include monoalkylaminoester (meth)acrylates such as monomethylaminoethyl (meth)acrylate, monoethylaminoethyl (meth)acrylate, monomethylaminopropyl (meth)acrylate, and monoethylaminopropyl (meth)acrylate.

Other monomers containing nitrogen atoms include (meth)acrylamide, N-vinylpyrrolidone, and acryloylmorpholine.

Examples of monomers having an epoxy group include glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl (meth)acrylate, and 6-methyl-3,4-epoxycyclohexylmethyl (meth)acrylate.

Examples of monomers having an isocyanato group include 2-isocyanatoethyl (meth)acrylate.

[Method of producing acrylic ester copolymer]
The method for producing the acrylic acid ester copolymer is not particularly limited, and can be obtained, for example, by polymerizing the above-mentioned monomer mixture using a known radical polymerization reaction. The reaction can be carried out in the absence of a solvent, but it is preferable to use a solvent from the viewpoints of synthesis stability and handling. Furthermore, it is preferable to use a radical polymerization initiator (hereinafter sometimes abbreviated as "polymerization initiator") from the viewpoint of molecular weight control. In addition, known additives such as chain transfer agents may be used.
As the solvent, the solvents described below can be used.

Examples of the polymerization initiator include benzoyl peroxide, tert-butyl perbenzoate, cumene hydroperoxide, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl)peroxydicarbonate, tert-butylperoxy-2-ethylhexanoate, tert-butylperoxyneodecanoate, tert-butylperoxypivalate, (3,5,5-trimethylhexanoyl)peroxide, dipropionyl peroxide, and diacetyl peroxide. Organic peroxides such as cyclohexane-1-carbonitrile, 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl 2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), 2,2'-azobis[2-(2-
Examples of the azo compounds include azo compounds such as (imidazolin-2-yl)propane.

Examples of chain transfer agents include alkyl mercaptans such as octyl mercaptan, nonyl mercaptan, decyl mercaptan, and dodecyl mercaptan; thioglycolic acid esters such as octyl thioglycolate, nonyl thioglycolate, and 2-ethylhexyl thioglycolate; 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, α-pinene, and β-pinene. Thioglycolic acid esters, 2,4-diphenyl-4-methyl-1-pentene, 1-methyl-4-isopropylidene-1-cyclohexene, α-pinene, and β-pinene are particularly preferred because the resulting polymers have a low odor.

The weight-average molecular weight Mw of the acrylic acid ester copolymer, as measured by gel permeation chromatography (GPC) relative to standard polystyrene, is preferably 200,000 to 2,000,000, more preferably 300,000 to 1,000,000, even more preferably 400,000 to 900,000, and particularly preferably 500,000 to 800,000. A weight-average molecular weight of 200,000 or more ensures sufficient cohesive strength. A weight-average molecular weight of 2,000,000 or less can suppress an increase in viscosity, ensuring good coating suitability.

The molecular weight dispersity Mw/Mn of the acrylic ester copolymer, calculated as a standard polystyrene equivalent by gel permeation chromatography (GPC), is preferably 2.0 to 12, and more preferably 4 to 10. Having a molecular weight dispersity within this range allows for a balance between adhesive strength and cohesive strength. Note that Mn refers to the number average molecular weight.

The glass transition temperature (Tg) of the acrylic ester copolymer is not particularly limited, but is preferably -70 to -25°C, and more preferably -65 to -30°C. A glass transition temperature of -70°C or higher can achieve sufficient cohesive strength. A glass transition temperature of -25°C or lower can demonstrate good adhesive strength.

In the present invention, the glass transition temperature (Tg) of the acrylic acid ester copolymer (A) is a value calculated based on the following formula (2) (Fox formula).
1/Tg=W1/Tg1+W2/Tg2+...+Wn/Tgn (Formula 2)
[In formula (1), Tg represents the Tg (unit: K) of the acrylic acid ester copolymer (A), Tgi (i = 1, 2, ..., n) represents the Tg (unit: K) when a homopolymer is formed from radically polymerizable monomer i, and Wi (i = 1, 2, ..., n) represents the mass fraction of radically polymerizable monomer i in all monomer components. Note that the Tg of the homopolymer uses a published value such as a literature value or catalog value.]
The above formula (2) is a calculation formula when the acrylic acid ester copolymer (A) is composed of n types of monomer components, namely, Monomer 1, Monomer 2, . . . , Monomer n.

<Isocyanate-based curing agent>
The pressure-sensitive adhesive composition of the present invention contains an isocyanate-based curing agent as a curing agent. The isocyanate-based curing agent forms crosslinks with the hydroxyl group-containing monomer (C) contained in the acrylic ester copolymer, thereby exhibiting high adhesive strength and cohesive strength.

Examples of the isocyanate curing agent include compounds having three or more isocyanate groups in the molecule, such as adducts of diisocyanates such as tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, diphenylmethane diisocyanate, hydrogenated diphenylmethane diisocyanate, tetramethylxylylene diisocyanate, naphthalene diisocyanate, triphenylmethane triisocyanate, and polymethylene polyphenyl isocyanate with polyol compounds such as trimethylolpropane, biuret compounds thereof, and isocyanurate compounds thereof, and adducts of the above diisocyanates with any of polyols such as polyether polyols, polyester polyols, acrylic polyols, polybutadiene polyols, and polyisoprene polyols; and compounds having two isocyanate groups in the molecule, such as allophanate compounds thereof. Among these, the trimethylolpropane adduct of tolylene diisocyanate, the trimethylolpropane adduct of xylylene diisocyanate, and the trimethylolpropane adduct of hexamethylene diisocyanate are preferred because their adhesive properties can be easily adjusted, and the trimethylolpropane adduct of tolylene diisocyanate is particularly preferred. Note that the number of isocyanate groups is the average number.

The amount of isocyanate curing agent is not particularly limited, but is preferably 0.1 to 10 parts by mass, more preferably 1 to 9 parts, and particularly preferably 1.5 to 7 parts, per 100 parts by mass of acrylic ester copolymer. An isocyanate curing agent content of 0.1 parts by mass or more provides sufficient cohesive strength, while an isocyanate curing agent content of 10 parts by mass or less can suppress zipping during peeling.

The pressure-sensitive adhesive composition of the present invention can further contain a curing agent other than an isocyanate-based curing agent. Examples of other curing agents include metal chelate-based curing agents, epoxy-based curing agents, and aziridine-based curing agents. The inclusion of these curing agents further increases the cohesive strength of the pressure-sensitive adhesive composition, improving its heat resistance.

Known metal chelate curing agents can be used, including, for example, coordination compounds of polyvalent metals such as aluminum, iron, copper, zinc, tin, titanium, nickel, antimony, magnesium, vanadium, chromium, and zirconium with ligands such as acetylacetone and ethyl acetoacetate. From the perspective of long-term storage stability, the polyvalent metal is preferably aluminum, and the ligand is preferably at least one of acetylacetone and acetoacetic ester.

Examples of the metal chelate curing agent include aluminum alkoxides such as aluminum isopropylate, monosec-butoxyaluminum diisopropylate, aluminum sec-butylate, and aluminum ethylate;
aluminum chelates such as ethyl acetoacetate aluminum diisopropylate, aluminum tris(ethyl acetoacetate), alkyl acetoacetate aluminum diisopropylate, aluminum monoacetylacetate bis(ethyl acetoacetate), and aluminum tris(acetylacetonate);
titanium alkoxides such as titanium tetraisopropoxide, titanium tetra-normal-butoxide, titanium butoxy dimer, and titanium tetra-2-ethylhexoxide;
titanium chelates such as titanium isopropoxybis(acetylacetonate), titanium tetraacetylacetonate, titanium diisopropoxybis(ethylacetoacetate), titanium phosphate ester complexes, and titanium octylene glycolate;
zirconium alkoxides such as zirconium tetra-normal propoxide and zirconium tetra-normal butoxide;
zirconium chelates such as zirconium tetraacetylacetonate, zirconium tributoxymonoacetylacetonate, zirconium tetraacetylacetonate, and zirconium dibutoxybis(ethylacetoacetate);
Examples include zirconium acylate such as zirconium stearate.

Among these, alkylacetoacetate aluminum diisopropylate, aluminum monoacetylacetate bis(ethylacetoacetate), and aluminum tris(acetylacetonate) are preferred from the viewpoints of crosslinkability, adhesive strength, transparency, and storage stability.

Examples of epoxy curing agents include bisphenol A-epichlorohydrin type epoxy resins, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, 1,6-hexanediol diglycidyl ether, trimethylolpropane triglycidyl ether, diglycidylaniline, N,N,N',N'-tetraglycidyl-m-xylylenediamine, 1,3-bis(N,N'-diglycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidylaminophenylmethane.

Examples of the aziridine curing agent include N,N'-diphenylmethane-4,4'-bis(1-aziridinecarboxite), N,N'-toluene-2,4-bis(1-aziridinecarboxite), bisisophthaloyl-1-(2-methylaziridine), tri-1-aziridinylphosphine oxide, N,N'-hexamethylene-1,6-bis(1-aziridinecarboxite), 2,2'-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate], trimethylolpropane tri-β-aziridinylpropionate, tetramethylolmethane tri-β-aziridinylpropionate, and tris-2,4,6-(1-aziridinyl)-1,3,5-triazine.

The content of other curing agents is not particularly limited, but is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less, per 100 parts by mass of the acrylic ester copolymer.

A known catalytic inhibitor may be added to the pressure-sensitive adhesive composition of the present invention for the purpose of improving the pot life of the pressure-sensitive adhesive.
Specific examples of the catalytic inhibitor include keto-enol tautomer-forming compounds, etc. Keto-enol tautomer-forming compounds are compounds that can form keto-enol tautomers, and preferred are acetylacetone, acetoacetic esters, malonic esters, etc.

Examples of acetoacetate esters include methyl acetoacetate, ethyl acetoacetate, isopropyl acetoacetate, butyl acetoacetate, isobutyl acetoacetate, and tert-butyl acetoacetate.

Examples of malonic acid esters include dimethyl malonate, diethyl malonate, and dibutyl malonate.
Among these, acetylacetone is preferred as the keto-enol tautomer-forming compound because it is easily volatilized when dried.

The content of the catalytic inhibitor is preferably 0.05 to 10.0 parts by mass, more preferably 0.1 to 5.0 parts by mass, and even more preferably 0.5 to 3.0 parts by mass, relative to 100 parts by mass of the acrylic acid ester copolymer. When the amount of the catalytic inhibitor is 0.05 part by mass or more, a good pot life is obtained, and when it is 10.0 parts by mass or less, good initial curing properties are obtained.

<Solvent>
The pressure-sensitive adhesive composition of the present invention may contain a solvent.
The timing of adding the solvent is not particularly important, and it may be added during the production of the acrylic ester copolymer, or it may be used as a dilution solvent to adjust the viscosity during the production of the pressure-sensitive adhesive composition. The solvent can be selected from the viewpoints of leveling and drying properties during coating, as well as effects on the environment and the human body. The solvent is preferably an aliphatic hydrocarbon, an alicyclic hydrocarbon, an aromatic hydrocarbon, an ester, a ketone, an alcohol, or the like.

Examples of solvents include ester-based solvents such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; ketone-based solvents such as acetone, methyl ethyl ketone, methyl butyl ketone, and cyclohexanone; hydrocarbon-based solvents such as toluene, xylene, and anisole; and alcohol-based solvents such as methanol, ethanol, normal propanol, and isopropanol. However, from the viewpoint of the pot life of the pressure-sensitive adhesive composition, it is preferable to include an ester-based solvent and/or an alcohol-based solvent.

The amount of solvent used is preferably 10 parts by weight or more, and more preferably 30 parts by weight or more, per 100 parts by weight of the acrylic ester copolymer. There is no particular upper limit, but from the standpoint of coatability, it is preferably 2,000 parts by weight or less.

<Tackifying Resin>
The pressure-sensitive adhesive composition of the present invention can further contain a tackifier resin. The tackifier resin can be used in any manner, such as by using it during solution polymerization or by blending it with an acrylic ester copolymer. When used during solution polymerization, the tackifier resin acts as a chain transfer agent, making it easier to adjust the molecular weight of the acrylic ester copolymer. Furthermore, when blended with an acrylic ester copolymer, the adhesive strength can be further improved.

The softening point of the tackifier resin is preferably 70°C to 170°C, more preferably 75°C to 160°C, and even more preferably 100°C to 150°C.
When the softening point of the tackifier resin is within the above range, it becomes easier to achieve high levels of both adhesive strength and cohesive strength. The softening point is the softening temperature measured according to the dry bulb method specified in JIS SK5902.

Examples of tackifying resins include, but are not limited to, rosin-based resins, polymerized rosin-based resins, rosin ester-based resins, polymerized rosin ester-based resins, terpene-based resins, terpene-phenol-based resins, coumarone-based resins, coumarone-indene-based resins, styrene-based resins, xylene-based resins, phenol-based resins, and petroleum-based resins. Among these, rosin-based resins, polymerized rosin-based resins, rosin ester-based resins, polymerized rosin ester-based resins, petroleum resins, and styrene-based resins are preferred because of their good compatibility with the acrylic ester copolymer (A) and improved adhesive performance. One type of tackifying resin may be used alone, or two or more types may be used in combination.

The amount of tackifier resin is preferably 1 to 60 parts by mass, more preferably 1.5 to 20 parts by mass, and even more preferably 2 to 10 parts by mass, per 100 parts by mass of acrylic ester copolymer. Adding 1 part by mass or more of tackifier resin can increase adhesive strength, while adding 60 parts by mass or less can maintain removability.

<Plasticizer>
The pressure-sensitive adhesive composition of the present invention may further contain a plasticizer, if necessary, for the purpose of suppressing zipping during peeling. The plasticizer is not particularly limited, but an organic acid ester is preferred from the viewpoint of compatibility with the acrylic acid ester copolymer, etc.

Examples of esters of monobasic acids or polybasic acids with alcohols include isostearyl laurate, isopropyl myristate, isocetyl myristate, octyldodecyl myristate, isostearyl palmitate, isocetyl stearate, octyldodecyl oleate, dibutyl phthalate, dioctyl phthalate, diheptyl phthalate, dibenzyl phthalate, butylbenzyl phthalate, diisodecyl adipate, diisostearyl adipate, dibutyl sebacate, diisocetyl sebacate, tributyl acetylcitrate, tributyl trimellitate, trioctyl trimellitate, trihexyl trimellitate, trioleyl trimellitate, and triisocetyl trimellitate.

Other examples of acid-alcohol esters include esters of unsaturated or branched fatty acids such as myristoleic acid, oleic acid, linoleic acid, linolenic acid, isopalmitic acid, and isostearic acid with alcohols such as ethylene glycol, propylene glycol, glycerin, trimethylolpropane, pentaerythritol, and sorbitan.

Examples of esters of monobasic or polybasic acids with polyalkylene glycols include polyethylene glycol dihexylate, polyethylene glycol di-2-ethylhexylate, polyethylene glycol dilaurate, polyethylene glycol dioleate, and dipolyethylene glycol methyl ether adipate.

Examples of phosphate ester plasticizers include tricresyl phosphate (TCP), triphenyl phosphate (TPP), tri-2-ethylhexyl phosphate (TOP), trixylenyl phosphate (TXP), and triethyl phosphate (TEP).

Epoxy plasticizers include epoxidized soybean oil, epoxidized linseed oil, epoxidized octyl stearate, epoxidized butyl fatty acid, and epoxidized butyl linseed oil fatty acid.

The molecular weight (formula weight or Mn) of the plasticizer is preferably 250 to 1000, more preferably 400 to 900, and particularly preferably 500 to 850. A molecular weight of 250 or higher will result in good heat resistance for the adhesive layer, while a molecular weight of 1000 or lower will provide sufficient zipping suppression.

There are no particular restrictions on the amount of plasticizer used, but it is preferably 0.01 to 30 parts by weight, and more preferably 4 to 20 parts by weight, per 100 parts by weight of the acrylic ester copolymer. A plasticizer content of 0.01 parts by weight or more will provide sufficient suppression of zipping during peeling, while a content of 30 parts by weight or less will prevent a decrease in adhesive strength.

<Antioxidants>
The pressure-sensitive adhesive composition of the present invention may further contain an antioxidant, if necessary.
Examples of antioxidants include radical scavengers and peroxide decomposers. Examples of radical scavengers include phenolic compounds and amine compounds. Examples of peroxide decomposers include sulfur compounds and phosphorus compounds.

Examples of phenolic compounds include 2,6-di-t-butyl-p-cresol, butylated hydroxyanisole, 2,6-di-t-butyl-4-ethylphenol, stearin-β-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 2,2'-methylenebis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), 4,4'-thiobis(3-methyl-6-t-butylphenol), 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 3,9-bis[1,1-dimethyl-2-[β-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraoxaspiro[5,5]undecane, benzophenone, benzophenone, benzophenone-4-hydroxybenzoates, benzophenone-5-hydroxybenzoates, benzophenone-6-hydroxybenzoates, benzophenone-7-hydroxybenzoates, benzophenone-8-hydroxybenzoates, benzophenone-9-hydroxybenzoates, benzophenone-10-hydroxybenzoates, benzophenone-11-hydroxybenzoates, benzophenone-12-hydroxybenzoates, benzophenone-13-hydroxybenzoates, benzophenone-14-hydroxybenzoates, benzophenone-15-hydroxybenzoates, benzophenone-16-hydroxybenzoates, benzophenone-17-hydroxybenzoates, benzophenone-18-hydroxybenzoates, benzophenone-19-hydroxybenzoates, benzophenone-20-hydroxybenzoates, benzophenone-21-hydroxybenzoates, benzophenone-22-hydroxybenzoates, benzophenone-23-hydroxybenzoates, benzophenone-24-hydroxybenzoates, benzophenone-25-hydroxybenzoates, benzophenone-26-hydroxybenzoates, benzophenone-27 Examples of such azenepropanoic acid esters include 3,5-bis(1,1-dimethylethyl)-4-hydroxy-, C7-C9 side chain alkyl esters, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, tetrakis-[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane, bis[3,3'-bis-(4'-hydroxy-3'-t-butylphenyl)butyric acid]glycol ester, 1,3,5-tris(3',5'-di-t-butyl-4'-hydroxybenzyl)-S-triazine-2,4,6-(1H,3H,5H)trione, and tocopherol.

Sulfur-based antioxidants include dilauryl 3,3'-thiodipropionate, dimyristyl 3,3'-thiodipropionate, and distearyl 3,3'-thiodipropionate.

Examples of phosphorus compounds include triphenyl phosphite, diphenyl isodecyl phosphite, 4,4'-butylidene-bis(3-methyl-6-tert-butylphenylditridecyl)phosphite, cyclic neopentanetetraylbis(octadecyl phosphite), tris(nonylphenyl)phosphite, tris(mononylphenyl)phosphite, tris(dinonylphenyl)phosphite, diisodecyl pentaerythritol diphosphite, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 10-(3,5-di-tert-butyl- 4-hydroxybenzyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 10-decyloxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene, tris(2,4-di-tert-butylphenyl)phosphite, cyclic neopentanetetraylbis(2,4-di-tert-butylphenyl)phosphite, cyclic neopentanetetraylbis(2,6-di-tert-butyl-4-methylphenyl)phosphite, and 2,2-methylenebis(4,6-di-tert-butylphenyl)octylphosphite.

By using an antioxidant, thermal deterioration of the acrylic acid ester copolymer can be prevented, and the heat resistance can be improved.
The amount of antioxidant added is not particularly limited, and is preferably 0.01 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, and even more preferably 0.2 to 2 parts by mass, per 100 parts by mass of the acrylic acid ester copolymer.

As the antioxidant, a phenolic compound, which is a radical scavenger, is preferred from the standpoints of transparency and polymerization inhibition. Phenol-based compounds can effectively prevent residual monomer components from generating low-molecular-weight components over time during the synthesis of the acrylic ester copolymer (A), which can reduce heat resistance.

The pressure-sensitive adhesive composition of the present invention may further contain known additives, such as catalysts, antistatic agents (AS agents), ultraviolet absorbers, light stabilizers, lubricants, corrosion inhibitors, heat stabilizers, weathering stabilizers, hydrolysis inhibitors, antifungal agents, thickeners, fillers (talc, calcium carbonate, titanium oxide, etc.), silane coupling agents, polymerization inhibitors, defoamers, slip agents, antiblocking agents, antifogging agents, lubricants, dyes, waxes, emulsions, magnetic materials, and dielectric property adjusters, as needed.

<Pressure-sensitive adhesive composition>
The pressure-sensitive adhesive composition of the present invention comprises an acrylic ester copolymer, which is a copolymer of a monomer mixture consisting of a (meth)alkyl acrylic ester monomer (A) having 1 to 12 carbon atoms in the alkyl group, a monomer (B) having an acidic group, and a monomer (C) having a hydroxyl group, and an isocyanate-based curing agent. The content of the acidic group-containing monomer (B) is from 0.01 to 6% by mass, and the content of the hydroxyl group-containing monomer (C) is from 0.05 to 10% by mass, based on 100% by mass of the monomer mixture. The gel fraction is 55% or more.

<Gel fraction>
The gel fraction is a value calculated by the following formula (3).
(Gel fraction)={(W2−W0−W3)/(W1−W0−W3)}×100 (Equation 3)
W0: Weight of wire mesh W1: Weight of wire mesh + test piece W2: Weight of wire mesh + test piece after drying W3: Weight of test piece substrate The method for measuring the gel fraction is described in detail in the Examples section.

The gel fraction of the pressure-sensitive adhesive composition of the present invention is 55% or more and 100% or less, preferably 70% or more and 98% or less, and more preferably 80% or more and 95% or less. A gel fraction of 55% or more ensures good removability even after storage at high temperatures.

<Viscoelasticity>
The storage modulus (G'), loss modulus (G'') and loss tangent (tan δ) of the pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition of the present invention were measured for temperature dependency using a viscoelasticity measuring device "Ares-G2" manufactured by TA Instruments Co., Ltd. under conditions of a frequency of 1 Hz, a strain of 0.1%, and a temperature of -50°C to 200°C (heating rate of 10°C/min), and the values at 25°C and 150°C were used.

The pressure-sensitive adhesive composition of the present invention has a ratio (P) of the loss modulus at 25°C to the loss modulus at 150°C when measured at 1 Hz, represented by Formula 1, of 0.02 to 0.3, preferably 0.03 to 0.15, more preferably 0.04 to 0.07, and particularly preferably 0.05 to 0.06. When the value of (P) is within this range, good adhesive strength and tack can be exhibited when applied, good removability can be exhibited even when applied for a long period of time under high-temperature conditions, and bleeding of the pressure-sensitive adhesive can be suppressed even when a load is applied at high temperatures.

(P) = (loss modulus at 150°C when measured at 1 Hz) / (25°C when measured at 1 Hz)
Loss modulus in °C) (Equation 1)

The storage modulus (G') at 25°C of the adhesive layer formed from the adhesive composition of the present invention is not particularly limited, but is preferably 15,000 Pa or more and 90,000 Pa or less, and more preferably 30,000 Pa or more and 65,000 Pa or less. A modulus of 15,000 Pa or more ensures sufficient cohesive strength, while a modulus of 90,000 Pa or less ensures good adhesive strength during use.

The storage modulus (G') at 150°C of the adhesive layer formed from the adhesive composition of the present invention is not particularly limited, but is preferably 10,000 Pa or more and 50,000 Pa or less. A modulus of 15,000 Pa or more results in an adhesive with little extrusion even at high temperatures, while a modulus of 50,000 Pa or less results in good adhesive strength even at high temperatures.

The loss modulus (G'') at 25°C of the pressure-sensitive adhesive layer formed from the pressure-sensitive adhesive composition of the present invention is not particularly limited, but is preferably from 10,000 Pa to 40,000 Pa, and more preferably from 15,000 Pa to 30,000 Pa. A loss modulus of 10,000 Pa or more ensures sufficient adhesive strength and tack, while a loss modulus of 40,000 Pa or less ensures good removability during use.

The loss modulus (G") at 150°C of the adhesive layer formed from the adhesive composition of the present invention is not particularly limited, but is preferably 800 Pa or more and 8,000 Pa or less, and more preferably 900 Pa or more and 2,500 Pa or less. A loss modulus of 800 Pa or more ensures sufficient adhesive strength and tack even at high temperatures, while a loss modulus of 8,000 Pa or less ensures good removability even at high temperatures.

<Method of manufacturing pressure-sensitive adhesive composition>
The method for producing the pressure-sensitive adhesive composition of the present invention is not particularly limited.
The pressure-sensitive adhesive composition of the present invention can be produced by adding and mixing an isocyanate-based curing agent, a solvent, and, if necessary, optional components to the acrylic acid ester copolymer.

The pressure-sensitive adhesive composition of the present invention preferably has a non-volatile content of 30 to 75% by mass, more preferably 38 to 65% by mass, and even more preferably 40 to 60% by mass. By setting the non-volatile content of the pressure-sensitive adhesive composition at a high level, the solvent content can be relatively reduced, thereby reducing drying costs during coating and minimizing the burden on the environment and human body.

The viscosity of the pressure-sensitive adhesive composition at 25°C is preferably 1,000 to 20,000 mPa·s, more preferably 1,500 to 10,000 mPa·s, and even more preferably 2,500 to 7,000 mPa·s. When the viscosity of the pressure-sensitive adhesive composition is within the above range, the coating properties are improved, and a pressure-sensitive adhesive layer with a smooth surface is more easily obtained. The viscosity is measured in an atmosphere of 25°C using a BL-type viscometer with a #3 rotor at a rotation speed of 12 rpm, one minute after the start of rotation.

<Adhesive sheet>
The pressure-sensitive adhesive sheet comprises a substrate and a pressure-sensitive adhesive layer made of a cured product of the pressure-sensitive adhesive composition of the present invention. The pressure-sensitive adhesive layer can be formed on one or both sides of the substrate. If necessary, the exposed surface of the pressure-sensitive adhesive layer can be covered with a release sheet. The release sheet is peeled off when the pressure-sensitive adhesive sheet is attached to an adherend.

When applying the adhesive composition, the viscosity can be adjusted using the solvents mentioned above, or the adhesive composition can be heated to reduce its viscosity.

The substrate is not particularly limited, and examples include resin sheets, paper, and metal foils. The substrate may also be a laminated sheet in which one or more layers are laminated on at least one surface of the substrate. If necessary, the surface of the substrate on which the adhesive layer is to be formed may be subjected to an adhesion-enhancing treatment such as corona discharge treatment or application of an anchor coating agent.

The constituent resin of the resin sheet is not particularly limited, and examples thereof include ester-based resins such as polyethylene terephthalate (PET); olefin-based resins such as polyethylene (PE) and polypropylene (PP); vinyl-based resins such as polyvinyl chloride; amide-based resins such as nylon 66; urethane-based resins (including foams); and combinations of these.
The thickness of the resin sheets excluding the polyurethane sheet is not particularly limited, but is preferably 15 to 300 μm. The thickness of the polyurethane sheet (including the foam) is not particularly limited, but is preferably 20 to 50,000 μm.

The paper is not particularly limited, and examples thereof include plain paper, coated paper, and art paper.
The constituent metal of the metal foil is not particularly limited, and examples thereof include aluminum, copper, and combinations thereof.

The release sheet is not particularly limited, and any known release sheet can be used, which has been subjected to a known release treatment such as coating the surface of a base sheet such as a resin sheet or paper.

The pressure-sensitive adhesive sheet can be produced by a known method.
First, the pressure-sensitive adhesive composition of the present invention is applied to the surface of a substrate to form a coating layer made of the pressure-sensitive adhesive composition of the present invention. As the application method, a known method can be applied, such as a roll coater method, a comma coater method, a die coater method, a reverse coater method, a silk screen method, or a gravure coater method.
The coating layer is then dried and cured to form a pressure-sensitive adhesive layer made of a cured product of the pressure-sensitive adhesive composition of the present invention. The heat-drying temperature is not particularly limited, but is preferably about 60 to 150°C. The thickness of the pressure-sensitive adhesive layer (thickness after drying) varies depending on the application, but is preferably 0.1 to 200 μm.
Next, if necessary, a release sheet is attached to the exposed surface of the adhesive layer by a known method.
In this manner, a single-sided PSA sheet can be produced.
By carrying out the above operation on both sides, a double-sided PSA sheet can be produced.

In contrast to the above method, the pressure-sensitive adhesive composition of the present invention may be applied to the surface of a release sheet to form a coating layer comprising the pressure-sensitive adhesive composition of the present invention, and the coating layer may then be dried and cured to form a pressure-sensitive adhesive layer comprising a cured product of the pressure-sensitive adhesive composition of the present invention, and a substrate sheet may be laminated on the exposed surface of the pressure-sensitive adhesive layer.

The method for producing a pressure-sensitive adhesive sheet preferably includes a coating step of coating a substrate with a pressure-sensitive adhesive, a heating step of subjecting the formed coating layer to a heat-drying treatment to form a pressure-sensitive adhesive layer containing a cured product of the pressure-sensitive adhesive composition, a winding step of winding the resulting pressure-sensitive adhesive sheet around a core to form a pressure-sensitive adhesive sheet roll, and a curing step of curing the pressure-sensitive adhesive sheet roll.

Synthesis examples, examples according to the present invention, and comparative examples are described below. In the following descriptions, unless otherwise specified, "parts" means parts by mass, "%" means % by mass, and "RH" means relative humidity.

[Measurement of weight average molecular weight (Mw)]
The weight average molecular weight (Mw) was measured by gel permeation chromatography (GPC) under the following conditions. Mw is a polystyrene equivalent value.
<Measurement conditions>
Apparatus: Shimadzu Corporation, LC-GPC system "Prominence"
Column: Four GMHXL columns manufactured by Tosoh Corporation and one HXL-H column manufactured by Tosoh Corporation connected in series.
Detector: differential refractive index detector (RID-10A),
Solvent: tetrahydrofuran (THF),
Flow rate: 1 mL/min,
Solvent temperature: 40°C,
Sample concentration: 0.2%,
Sample injection volume: 100 μL

<Production of Acrylic Ester Copolymer>
(Synthesis Example 1)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid,
After charging 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), the air in the reaction vessel was replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.2 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out at reflux temperature under a nitrogen atmosphere for 8 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic ester copolymer was 790,000.

(Synthesis Example 2)
A reaction vessel (hereinafter sometimes simply referred to as the "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 4-hydroxybutyl acrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 4-hydroxybutyl acrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.2 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. The contents of the reaction vessel were then heated to 80°C under a nitrogen atmosphere with stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. At the same time, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic ester copolymer was
was 770,000.

(Synthesis Example 3)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.84 parts of 2-carboxyethyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN).
2 parts of butyl acrylate, 16.4 parts of butyl acrylate, 1.96 parts of 2-carboxyethyl acrylate, and 5 parts of ethyl acetate were charged into vessel A, and vessel B was charged with 16.4 parts of butyl acrylate, 1.96 parts of 2-carboxyethyl acrylate, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after initiating the reaction, the contents of vessel A were added dropwise over 2 hours using a dropping pump. At the same time, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. A polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 820,000.

(Synthesis Example 4)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 7.2 parts of butyl acrylate, 10 parts of ethyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after initiating the reaction, the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 880,000.

(Synthesis Example 5)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 12 parts of 2-ethylhexyl acrylate, 7.5 parts of isononyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, vessel A equipped with a stirrer was charged with 28 parts of 2-ethylhexyl acrylate, 17.5 parts of isononyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.2 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere, and after initiating the reaction, the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 810,000.

(Synthesis Example 6)
Into a reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube, 30 parts of 2-ethylhexyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
After charging 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), the air in the reaction vessel was replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 19.15 parts of butyl acrylate, 0.05 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. A polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 800,000.

(Synthesis Example 7)
Into a reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube, 30 parts of 2-ethylhexyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
After charging 0.02 parts of 2-ethylhexyl acrylate and 0.15 parts of lauryl methacrylate into a vessel A equipped with a stirrer, the air in the vessel was replaced with nitrogen gas.
18.4 parts of butyl acrylate, 0.8 parts of acrylic acid, and 5 parts of ethyl acetate were charged to vessel B. Next, vessel A was charged with 18.4 parts of butyl acrylate, 0.8 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 800,000.

(Synthesis Example 8)
Into a reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube, 30 parts of 2-ethylhexyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
After charging 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), the air in the reaction vessel was replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 18.2 parts of butyl acrylate, 1 part of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic ester copolymer was 800,000.

(Synthesis Example 9)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.84 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and a vessel B was charged with 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN).
To the reaction vessel were charged 16.4 parts of butyl acrylate, 1.96 parts of acrylic acid, and 5 parts of ethyl acetate. The contents of the reaction vessel were then heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic ester copolymer was 810,000.

(Synthesis Example 10)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 1.05 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and a vessel B
To the reaction vessel were charged 15.7 parts of butyl acrylate, 2.45 parts of acrylic acid, and 5 parts of ethyl acetate. The contents of the reaction vessel were then heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic ester copolymer was 820,000.

(Synthesis Example 11)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 1.65 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 13 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and a vessel B was charged with 2-ethylhexyl acrylate, 1.65 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN).
To the reaction vessel were charged 15.7 parts of butyl acrylate, 3.85 parts of acrylic acid, and 5 parts of ethyl acetate. The contents of the reaction vessel were then heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic ester copolymer was 850,000.

(Synthesis Example 12)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.02 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.03 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.95 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 790,000.

(Synthesis Example 13)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.04 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.03 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.93 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 790,000.

(Synthesis Example 14)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.08 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.07 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.85 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 780,000.

(Synthesis Example 15)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.5 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then purged with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 17.1 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 790,000.

(Synthesis Example 16)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.8 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 0.7 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 16.5 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 790,000.

(Synthesis Example 17)
A reaction vessel (hereinafter sometimes simply referred to as the "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 35 parts of 2-ethylhexyl acrylate, 11 parts of lauryl methacrylate, 2 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 16 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. The contents of the reaction vessel were then heated to 80°C under a nitrogen atmosphere with stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. At the same time, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out at reflux temperature for 8 hours under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic acid ester copolymer was 810,000.

(Synthesis Example 18)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 65 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 15 parts of 2-ethylhexyl acrylate, 5 parts of lauryl methacrylate, 32.2 parts of butyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN).
0.02 parts of butyl acrylate, and 15 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate were charged into vessel A. The reaction vessel was then heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic ester copolymer was 830,000.

(Synthesis Example 19)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 65 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 15 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 22.2 parts of butyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 10 parts of ethyl acetate, and 2,2'-azobisisobutyronitrile (AIBN).
0.02 parts of butyl acrylate, 15 parts of acrylic acid, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate were charged into vessel A. Next, the reaction vessel was heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic ester copolymer was 800,000.

(Synthesis Example 20)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 65 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 22.2 parts of 2-ethylhexyl acrylate, 15 parts of lauryl methacrylate, 15 parts of butyl acrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 10 parts of ethyl acetate, and 2,2'-azobisisobutyronitrile (AIBN).
0.02 parts of butyl acrylate, 15 parts of acrylic acid, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate were charged into vessel A. Next, the reaction vessel was heated to 80°C with stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic ester copolymer was 770,000.

(Synthesis Example 21)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 40 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 34 parts of 2-ethylhexyl acrylate, 5 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 35 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 28.2 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 700,000.

(Synthesis Example 22)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 45 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then purged with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 15 parts of 2-ethylhexyl acrylate, 42 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 20 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 10.2 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 720,000.

(Synthesis Example 23)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 30 parts of 2-ethylhexyl acrylate, 0.6 parts of acrylic acid, 0.4 parts of 2-hydroxyethyl methacrylate, 40 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Furthermore, vessel A equipped with a stirrer was charged with 16 parts of 2-ethylhexyl acrylate, 36 parts of lauryl methacrylate, 0.4 parts of 2-hydroxyethyl methacrylate, 5 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and vessel B was charged with 10 parts of lauryl methacrylate, 5.2 parts of butyl acrylate, 1.4 parts of acrylic acid, and 5 parts of ethyl acetate. Next, the reaction vessel was heated to 80°C under a nitrogen atmosphere while stirring to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. Simultaneously, the contents of vessel B were fed to vessel A over 2.0 hours with stirring. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 800,000.

(Synthesis Example 24)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 13.5 parts of 2-ethylhexyl acrylate, 1.5 parts of lauryl methacrylate, 14.2 parts of butyl acrylate, 0.6 parts of acrylic acid, 0.24 parts of 2-hydroxyethyl methacrylate, 50 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 31.5 parts of 2-ethylhexyl acrylate, 3.5 parts of lauryl methacrylate, 33 parts of butyl acrylate, 1.4 parts of acrylic acid, 0.56 parts of 2-hydroxyethyl methacrylate, 30 parts of ethyl acetate, and 2,2'-azobisisobutyronitrile (AIBN).
0.02 parts of toluene (IBN) was charged. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise using a dropping pump over 2 hours. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 830,000.

(Synthesis Example 25)
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet tube was charged with 8.4 parts of 2-ethylhexyl acrylate, 0.9 parts of lauryl methacrylate, 14 parts of butyl acrylate, 6 parts of ethyl acrylate, 0.6 parts of acrylic acid, 0.3 parts of 2-hydroxyethyl methacrylate, 50 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN). The air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 19.6 parts of 2-ethylhexyl acrylate, 32.9 parts of butyl acrylate, 2.1 parts of lauryl methacrylate, 14 parts of ethyl acrylate, 1.4 parts of acrylic acid, 0.7 parts of 2-hydroxyethyl methacrylate, 30 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN). Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. The polymerization reaction was carried out at reflux temperature under a nitrogen atmosphere for 8 hours. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic acid ester copolymer was 700,000.

Comparative Synthesis Example 1
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 13.5 parts of 2-ethylhexyl acrylate, 1.5 parts of lauryl methacrylate, 14 parts of butyl acrylate, 0.8 parts of acrylic acid, 60 parts of ethyl acetate, 2,
After charging 0.02 parts of 2'-azobisisobutyronitrile (AIBN), the air in the reaction vessel was replaced with nitrogen gas. Further, in a vessel A equipped with a stirrer, 31.5 parts of 2-ethylhexyl acrylate, 33.2 parts of butyl acrylate, 3.5 parts of lauryl methacrylate, 2.0 parts of acrylic acid, 30 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (AIBN),
0.02 parts of acetone was charged. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 810,000.

Comparative Synthesis Example 2
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 13.5 parts of 2-ethylhexyl acrylate, 1.5 parts of lauryl methacrylate, 14 parts of butyl acrylate, 0.6 parts of 2-hydroxyethyl methacrylate, 60 parts of ethyl acetate, and 0.02 parts of 2,2'-azobisisobutyronitrile (AIBN), and the air in the reaction vessel was then replaced with nitrogen gas. Further, a vessel A equipped with a stirrer was charged with 31.5 parts of 2-ethylhexyl acrylate, 34 parts of butyl acrylate, 3.5 parts of lauryl methacrylate, 1.4 parts of 2-hydroxyethyl methacrylate, 30 parts of ethyl acetate, and 2,2'
0.02 parts of azobisisobutyronitrile (AIBN) was then charged. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 810,000.

Comparative Synthesis Example 3
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 13.5 parts of 2-ethylhexyl acrylate, 1.5 parts of lauryl methacrylate, 10.5 parts of butyl acrylate, 2.7 parts of acrylic acid, 1.8 parts of 2-hydroxyethyl methacrylate, 55 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (
After charging 0.02 parts of 2-ethylhexyl acrylate, the air in the reaction vessel was replaced with nitrogen gas. Furthermore, in a vessel A equipped with a stirrer, 31.5 parts of 2-ethylhexyl acrylate, 24.5 parts of butyl acrylate, 3.5 parts of lauryl methacrylate, 6.3 parts of acrylic acid, 4.2 parts of 2-hydroxyethyl methacrylate, 15 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
0.02 parts of toluene (IBN) was charged. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise using a dropping pump over 2 hours. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 880,000.

Comparative Synthesis Example 4
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 13.5 parts of 2-ethylhexyl acrylate, 1.5 parts of lauryl methacrylate, 9.3 parts of butyl acrylate, 1.2 parts of acrylic acid, 4.5 parts of 2-hydroxyethyl methacrylate, 55 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
After charging 0.02 parts of 2,2'-azobisisobutyronitrile (A), the air in the reaction vessel was replaced with nitrogen gas. Furthermore, in a vessel A equipped with a stirrer, 31.5 parts of 2-ethylhexyl acrylate, 21.7 parts of butyl acrylate, 3.5 parts of lauryl methacrylate, 2.8 parts of acrylic acid, 10.5 parts of 2-hydroxyethyl methacrylate, 10 parts of ethyl acetate, and 2,2'-azobisisobutyronitrile (A
Next, 0.02 parts of 8-IBN was added to the reaction vessel under a nitrogen atmosphere while stirring.
The mixture was heated to 0°C to initiate the reaction, and the contents of vessel A were added dropwise over 2 hours using a dropping pump. The polymerization reaction was carried out at reflux temperature for 8 hours under a nitrogen atmosphere. After the reaction was completed, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a nonvolatile content of 35%. The weight-average molecular weight (Mw) of the acrylic acid ester copolymer was 850,000.

Comparative Synthesis Example 5
A reaction vessel (hereinafter sometimes simply referred to as "reaction vessel") equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube was charged with 13.5 parts of 2-ethylhexyl acrylate, 1.5 parts of lauryl methacrylate, 7.8 parts of butyl acrylate, 2.7 parts of acrylic acid, 4.5 parts of 2-hydroxyethyl methacrylate, 55 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
After charging 0.02 parts of 2-ethylhexyl acrylate, the air in the reaction vessel was replaced with nitrogen gas. Furthermore, in a vessel A equipped with a stirrer, 31.5 parts of 2-ethylhexyl acrylate, 18.2 parts of butyl acrylate, 3.5 parts of lauryl methacrylate, 6.3 parts of acrylic acid, 10.5 parts of 2-hydroxyethyl methacrylate, 10 parts of ethyl acetate, 2,2'-azobisisobutyronitrile (A
0.02 parts of toluene (IBN) was charged. Next, the reaction vessel was heated to 80°C while stirring under a nitrogen atmosphere to initiate the reaction, and the contents of vessel A were added dropwise using a dropping pump over 2 hours. The polymerization reaction was carried out for 8 hours at reflux temperature under a nitrogen atmosphere. After completion of the reaction, the mixture was cooled and diluted with ethyl acetate to obtain an acrylic acid ester copolymer solution with a non-volatile content of 35%. The weight average molecular weight (Mw) of the acrylic acid ester copolymer was 870,000.

The abbreviations in Table 1 are as follows:
2EHA: 2-ethylhexyl acrylate INA: isononyl acrylate LMA: lauryl methacrylate EA: ethyl acrylate
BA: butyl acrylate
AA: acrylic acid BCEA: 2-carboxyethyl acrylate HEMA: 2-hydroxyethyl methacrylate 4HBA: 4-hydroxybutyl acrylate

Example 1
<Production of Pressure-Sensitive Adhesive Composition>
2.5 parts of TDI-TMP was blended as an isocyanate curing agent with 100 parts of the acrylic acid ester copolymer (Synthesis Example 1) in terms of non-volatile content, and ethyl acetate was further added as a solvent so that the non-volatile content became 30%, followed by mixing to obtain a uniform pressure-sensitive adhesive composition.

<Production of Pressure-Sensitive Adhesive Sheet>
The obtained pressure-sensitive adhesive composition was applied to the release-treated surface of a 38 μm-thick release sheet (made of polyethylene terephthalate) (hereinafter referred to as "release film substrate") so that the thickness after drying would be 25 μm, and the applied layer was dried in a hot air oven at 100°C for 90 seconds to produce a pressure-sensitive adhesive layer. After drying, the resulting layer was laminated onto a 50 μm-thick substrate (made of polyethylene terephthalate) and further aged at 23°C and 50% RH for 7 days to obtain a pressure-sensitive adhesive sheet.

(Examples 2 to 32, Comparative Examples 1 to 7)
Each pressure-sensitive adhesive composition and pressure-sensitive adhesive sheet were produced in the same manner as the pressure-sensitive adhesive sheet of Example 1, except that the acrylic acid ester copolymer and various additives were changed to the compositions and blending amounts (parts by mass) shown in Table 2.
However, Examples 5, 21, 22, and 27 to 32 are reference examples.

The materials used are as follows:
<Isocyanate-based curing agent>
TDI-TMP: Adduct of tolylene diisocyanate and trimethylolpropane <Other curing agents>
Azd: Aziridine-based curing agent, Chemitite PZ-33, manufactured by Nippon Shokubai Co., Ltd. (plasticizer)
ATBC: acetyl tributyl citrate

[Evaluation items and evaluation methods]
The resulting pressure-sensitive adhesive compositions and pressure-sensitive adhesive sheets were evaluated by the following methods.

(Gel fraction)
After coating, the adhesive sheet was aged for 7 days in an atmosphere of 23°C and 50% RH, and a test piece 30 mm wide and 100 mm long was cut out from the sheet. The release sheet was peeled off from the test piece, and the exposed adhesive layer was attached to a 300-mesh stainless steel wire mesh. The wire mesh was folded to prevent the test piece from peeling off, and the test piece was immersed in ethyl acetate while wrapped in the wire mesh and left to stand at 50°C for 24 hours. After immersion, the wire mesh was removed, washed with a small amount of ethyl acetate, dried at 100°C for 30 minutes, and then its weight was measured. The gel fraction was calculated using the following formula (3):
(Gel fraction)={(W2−W0−W3)/(W1−W0−W3)}×100 (Equation 3)
W0: Weight of wire mesh W1: Weight of wire mesh + test piece W2: Weight of wire mesh + test piece after drying W3: Weight of test piece substrate

(viscoelasticity)
The pressure-sensitive adhesive composition obtained above was coated onto the release-treated surface of a 38 μm-thick release sheet (made of polyethylene terephthalate) (hereinafter referred to as the "release film substrate") so that the thickness after drying would be 25 μm, and the coated film was dried in a hot air oven at 100°C for 90 seconds to produce a pressure-sensitive adhesive layer. After drying, the coated film was laminated onto another 75 μm-thick release sheet (made of polyethylene terephthalate) (hereinafter referred to as the "release film substrate") and aged for 7 days in an atmosphere of 23°C and 50% RH to produce a pressure-sensitive adhesive layer sandwiched between two release sheets in a "release sheet/pressure-sensitive adhesive layer/release sheet" configuration. Next, one release sheet was peeled off, and lamination was repeated so that the pressure-sensitive adhesive layers overlapped, resulting in a pressure-sensitive adhesive layer with a thickness of 1 mm. After removing air bubbles in an autoclave, the coated film was cut into a cylindrical shape with a diameter of 8 mm to prepare a test specimen for measuring the storage modulus. The release sheets on both sides were peeled off, and viscoelasticity was measured. The storage modulus (G'), loss modulus (G") and loss tangent (tan δ) of the pressure-sensitive adhesive layer were measured for temperature dependency using an Ares-G2 viscoelasticity measuring device manufactured by TA Instruments Co., Ltd. under conditions of a frequency of 1 Hz, a strain of 0.1%, and a temperature of -50°C to 200°C (heating rate of 10°C/min). Using the obtained loss modulus (G") values at 25°C and 150°C, the value of P was calculated from the following (Equation 1):
P = (loss modulus at 150°C when measured at 1 Hz) / (loss modulus at 25°C when measured at 1 Hz) (Equation 1)

(Adhesive strength)
After aging for 5 hours in an atmosphere of 23°C and 50% RH, a test piece measuring 25 mm in width and 150 mm in length was cut out from the pressure-sensitive adhesive sheet. The release sheet was peeled off from the cut-out pressure-sensitive adhesive sheet, and the exposed pressure-sensitive adhesive layer was attached to the surface of a stainless steel plate (SUS304) and pressed with a 2 kg roll back and forth once. The sample was then left in an atmosphere of 23°C and 50% RH for 24 hours. Next, in accordance with JIS Z0237, the adhesive strength (adhesive strength before heating) was measured using a tensile tester (Tensilon, manufactured by Orientec Co., Ltd.) at a peel speed of 300 mm/min and a peel angle of 180°. The evaluation criteria were as follows:
[Evaluation criteria]
◎: 3N/25mm or more, excellent.
○: 1N/25mm or more and less than 3N/25mm, good.
△: 0.6 N/25 mm or more and less than 1 N/25 mm, usable.
×: Less than 0.6N/25mm, not practical.

(Ball tuck)
The resulting adhesive sheet was prepared in a size of 25 mm wide and 250 mm long. The release sheet was peeled off from the adhesive sheet, and the sheet was fixed with the adhesive side facing up on an inclined plate with an inclination angle of 30 degrees. A PET film for the runway was attached to the top, and a steel ball (1/32 to 32/32 inches) was rolled across the sample, which had a runway of 10 cm and an adhesive surface of 10 cm, and the diameter number of the ball that stopped near the center of the adhesive surface was recorded. Measurements were carried out in an atmosphere of 23°C and 50% relative humidity.
Evaluation criteria under an atmosphere of 23°C and 50% relative humidity: Excellent: Ball diameter is 11 or more.
○: Ball diameter is 8 to 10. Good.
△: Ball diameter is 5 to 7. Practical.
×: Ball diameter is 4 or less. Not practical.

(Removal test)
After aging for 7 days in an atmosphere of 23°C-50% RH, a test piece measuring 25 mm in width and 150 mm in length was cut out from the pressure-sensitive adhesive sheet. The release sheet was peeled off from the cut-out pressure-sensitive adhesive sheet, and the exposed pressure-sensitive adhesive layer was attached to the surface of a stainless steel plate (SUS304) and pressed back and forth with a 2 kg roll. The test piece was then stored for 10 days in environments of 23°C-50% RH, 40°C, 60°C, and 55°C-80% RH. After 10 days, the test piece was left at 23°C-50% RH for 3 hours. Then, in accordance with JIS Z0237, the adhesive strength (adhesive strength before heating) was measured using a tensile tester (Tensilon, manufactured by Orientec Co., Ltd.) at a peel rate of 300 mm/min and a peel angle of 180°, and the condition of the stainless steel plate after measurement was observed. The evaluation criteria are as follows.
[Evaluation criteria]
◎: No adhesive adhered to the stainless steel plate, excellent.
◯: Adhesion of the adhesive to the stainless steel plate was more than 0% and less than 3% of the adhesion area, good.
△: Adhesion of the adhesive to the stainless steel plate was 3% or more and less than 5% of the adhesion area, and practically usable.
×: Adhesion of adhesive to the stainless steel plate was 5% or more of the adhesion area, and the tape was unusable.

(Protrusion test)
After aging for 5 hours or 7 days in an atmosphere of 23°C and 50% RH, pressure-sensitive adhesive sheets measuring 10 mm wide and 10 mm long were prepared. The release sheet was peeled off from the pressure-sensitive adhesive sheet, and the sheet was sandwiched between 38 μm-thick release sheets (made of polyethylene terephthalate) on the top and bottom. A pressure of 50 kg/ ^cm2 was applied using a press tester in an environment of 60°C, and the sheet was left for 2 hours. After the test was completed, the sample was removed and the width of the adhesive protruding from the edge of the test piece (maximum value) was measured.
Evaluation criteria: ⊚: Protrusion is less than 0.2 mm. Very good.
○: Protrusion is 0.2 mm or more and less than 0.4 mm. Good.
△: Protrusion is 0.4 to less than 0.6 mm. Practical use is possible.
×: Protrusion of 0.6 mm or more. Not practical.

(Storage stability)
After blending the curing agent, the pressure-sensitive adhesive composition was mixed uniformly and allowed to stand in an atmosphere of 23° C. and 50% RH, and the rate of increase in viscosity was calculated using the following formula (4).
(Viscosity increase rate) = {(Viscosity 7 days after adding the curing agent) / (Viscosity 1 hour after adding the curing agent)}
×100} (Formula 4)
[Evaluation criteria]
◎: Viscosity increase rate is less than 150%, excellent.
Good: Viscosity increase rate of 150% or more and less than 160%.
△: Viscosity increase rate is 160% or more but less than 170%, practically usable.
×: Viscosity increase rate is 170% or more, unusable.

The adhesive composition of the present invention comprises an acrylic acid ester copolymer, which is a copolymer of a monomer mixture consisting of a (meth)acrylic acid ester monomer (A) having 1 to 12 carbon atoms in the alkyl group, a monomer (B) having an acidic group, and a monomer (C) having a hydroxyl group, and an isocyanate-based curing agent, wherein the content of the monomer (B) having an acidic group is from 0.01 to 6 mass% based on 100 mass% of the monomer mixture, and the content of the monomer (C) having a hydroxyl group is from 0.05 to 10 mass% based on 100 mass% of the monomer mixture, and the gel fraction is 55% or more. All of the adhesive compositions of the present invention have good adhesive properties, good removability after long-term storage under various conditions, little adhesive extrusion under high temperature and high load, and adhesive sheets with excellent storage stability after blending with the curing agent were successfully produced.

Claims (4)

The composition includes an acrylic acid ester copolymer, which is a copolymer of a monomer mixture including a (meth)acrylic acid alkyl ester monomer (A) having 1 to 12 carbon atoms in the alkyl group, a monomer (B) having an acidic group, and a monomer (C) having a hydroxyl group, and an isocyanate-based curing agent;
the content of the (meth)acrylic acid alkyl ester monomer (a1) having an alkyl group with a carbon number of 8 to 12 is 55% by mass or more and 90% by mass or less in 100% by mass of the (meth)acrylic acid alkyl ester monomer (A) having an alkyl group with a carbon number of 1 to 12,
the content of the monomer (B) having an acidic group is 0.01% by mass or more and 6% by mass or less, based on 100% by mass of the monomer mixture;
the content of the hydroxyl group-containing monomer (C) is 0.05% by mass or more and 10% by mass or less, based on 100% by mass of the monomer mixture;
the isocyanate-based curing agent contains a trimethylolpropane adduct of tolylene diisocyanate;
The gel fraction is 55% or more,
The (meth)acrylic acid alkyl ester monomer (a1) having an alkyl group with 8 to 12 carbon atoms comprises 2-ethylhexyl acrylate and lauryl methacrylate . 2. The pressure-sensitive adhesive composition according to claim 1, wherein the ratio (P) of the loss modulus at 25°C to the loss modulus at 150°C measured at 1 Hz, as represented by Equation 1, is 0.02 or more and 0.3 or less.
P = (loss modulus at 150°C when measured at 1 Hz) / (loss modulus at 25°C when measured at 1 Hz) (Equation 1) 2. The pressure-sensitive adhesive composition according to claim 1, wherein the ratio (P) of the loss modulus at 25°C to the loss modulus at 150°C measured at 1 Hz, as represented by Equation 1, is 0.03 or more and 0.15 or less.
P = (loss modulus at 150°C when measured at 1 Hz) / (loss modulus at 25°C when measured at 1 Hz) (Equation 1) A pressure-sensitive adhesive sheet comprising a substrate and a pressure-sensitive adhesive layer comprising a cured product of the pressure-sensitive adhesive composition described in any one of claims 1 to 3.